Due to their properties, phosphorescent transition metal complexes become more and more important as highly efficient emitters in optoelectronic components such as OLEDs. The spin-orbit coupling induced by the transition metal atom (heavy metal atom) results in an increased intersystem-crossing rate from the excited singlet state to the triplet state and thus in the use of the singlet excitons as well as the triplet excitons for emission and thereby allows a theoretical achievable internal quantum yield of 100%.
These phosphorescent dyes are usually introduced into appropriate energetically adjusted host materials. Polymeric structures are particularly suitable for this purpose due to the ease of processing by liquid processing from solution. Ideally, these should fulfill additional functions such as the spatial separation of the dye molecules to prevent undesirable concentration quenching processes and triplet-triplet-annihilation under emission reduction, increased charge carrier injection and transport and an increased recombination probability directly on the emitter molecules.
Thus, the combination of suitable polymeric host structures with appropriate statistically blended emitter compounds and additionally inserted charge transport molecules represents a method diversely used for the preparation of polymeric light emitting diodes (PLEDs). Even though the OLED components produced this way have mostly high efficiencies, these mixed systems can be subject to undesired phase separations, aggregations or crystallization processes, which have a negative effect on the capacity and the lifetime of the components. Therefore, the production of adapted (co)polymers, which fulfill different functions such as charge transport and emission while at the same time using the advantages of liquid processing, is of steadily increasing interest.
For the synthesis of phosphorescent polymers with directly attached transition metal complexes, two different routes are in principle available in the prior art: on the one hand, the attachment of the metal complexes to the polymers provided with functional groups, which were prepared before (“complexation at the polymer”), and on the other hand, the polymerization of corresponding monomers, which carry the metal complexes (“polymerization of complex monomers”).
The first strategy allows a modular design with the basic attachment of a large amount of different metal complexes to the polymer and has as an advantage the extensive and more detailed analysis of the metal-free polymers synthesized before by common polymer analysis such as, for example, GPC and NMR. Additionally, the amount of metal complex in the final polymer can theoretically be varied by careful adjustment of the potential coordination sites. The use of functionalization methods orthogonal to the actual polymerization reactions, which also have to proceed in high yields, is necessary for the success of the modular post-polymerization method.
The advantage of the second route consists of the controlled structure and quantitative functionalization of the metal complexes by using common polymerization methods, which in part must be adjusted to the correspondent metal complex-functionalized monomers and whose accurate characterization by common analytical methods is not possible in most cases due to the attached metal complexes.
Both methods have in common that the efficient emitter complexes are attached to a polymeric host system and can thus can be applied to liquid processing; nevertheless, they are subject to the disadvantages of a possible multi-layer arrangement: The low-cost liquid-processing of polymers allows no simple sequential application of defined, thin layers. This is due to the general solubility of the polymeric materials. A relatively high amount of solvent is necessary for the desired material thickness and already dried layers are partly dissolved again during the application of subsequent layers, whereby the necessary layer arrangement is broken again.
Previous solutions to this problem are the development of cross-linkable materials with negative photoresist-like properties, which are cross-linked after deposition out of solution by exposure to light or thermal treatment, and thus form insoluble layers. Fréchet and co-workers reported, for example, on a number of cross-linkable heteroleptic Ir(III) complexes for the application in liquid-processable phosphorescent OLEDs, which carry two cross-linkable vinylbenzyl ether units, which can be fully cross-linked by heating to 180° C. (Multifunctional Crosslinkable Iridium Complexes as Hole Transporting/Electron Blocking and Emitting Materials for Solution-Processed Multilayer Organic Light-Emitting Diodes, Biwu Ma, Bumjoon J. Kim, Daniel A. Poulsen, Stefan J. Pastine, Jean M. J. Fréchet, Adv. Funct. Mater. 2009, 19, 1024-1031). The cross-linked films show high solvents resistance and very good properties for the formation of films, making the principle preparation of multi-layer systems by sequential liquid processing of different layers possible. However, this approach represents no controlled build-up of well-defined metal complex-functionalized polymers since the polymerization proceeds only by thermal processes and completely uncontrolled. It is, for example, not possible to exactly adjust by controlled polymerization methods the molecular weight, the chain length, and the polydispersity of the polymer to operate reproducibly and to make adjustments according to the requirements of a standardized liquid-processing.